diff --git a/repo2docker/docker.py b/repo2docker/docker.py
index 8fb35011..e2eeac06 100644
--- a/repo2docker/docker.py
+++ b/repo2docker/docker.py
@@ -75,7 +75,6 @@ class DockerEngine(ContainerEngine):
     )
 
     extra_buildx_build_args = List(
-        [Unicode],
         [],
         help="""
         Extra commandline arguments to pass to `docker buildx build` when building the image.